1. Field of the Invention
The present invention relates to a carrier wave reproducer device for use in the reception of a multi-phase digital phase-modulated wave suited particularly for the reception of a phase-modulated wave in a burst form.
2. Description of the Prior Art
In a digital radio communication system, it has been known that an N-phase (N being a positive integer of two or more) digital phase-modulation system is a system superior to other transmission systems. As a demodulation system for the N-phase digital phase-modulated wave, there are known a synchronized detection system in which a reference carrier wave synchronized in phase with the carrier wave of the input phase-modulated wave is reproduced on the receiver side and the demodulation is achieved through synchronized detection by making use of said reproduced carrier wave, and a delayed detection system in which the input digital phase-modulated wave is compared in phase with a delayed input modulated wave which has been shifted on a time axis by one time slot with respect to the first said input digital phase-modulated wave and thereby the demodulation is achieved. The former system has the disadvantage that it requires a reference carrier wave reproducer circuit although it is more excellent in performance against noise, while the latter system has the advantage that it does not require the reference carrier wave reproducer circuit although its performance against noise is poor. In addition, in contrast to the fact that the signal detected by the former system corresponds to the modulation phase in every time slot of the input phase-modulated wave, the signal detected by the latter system corresponds to the modulation phase difference between the successive time slots of the input phase-modulated wave. Consequently, the signal detected by the latter system is correlated to the signal demodulated by the former system in such a manner that the signals detected by the latter system in the successive time slots are added by means of an adder and thereby equalized to the signal demodulated by the former system.
Among the reference carrier wave reproducer circuits for reproducing a carrier wave to be applied to a demodulator in said synchronized detection system, an inverse modulation system and a frequency multiplier system have been heretofore known as representative ones. The inverse modulation system is constructed in such a manner that there are provided an N-phase phase-demodulator adapted to demodulate the input N-phase phase-modulated wave by means of the reproduced reference carrier wave, and an N-phase inverse phase-modulator adapted to cancel the phase shift of the input N-phase phase-modulated wave with the output of said demodulator, and that the reference carrier wave may be obtained from the output of said inverse modulator. On the other hand, the frequency multiplier system is constructed in such a manner that the input N-phase phase-modulated wave is multiplied in frequency by a factor of N by means of a frequency multiplier to derive an unmodulated wave having a frequency N times as high as the carrier frequency from the output of said frequency multiplier, and thereafter a reference carrier wave having a frequency equal to the input carrier frequency may be reproduced by means of a 1/N frequency divider.
In recent applications of digital radio communication techniques to satellite communication systems and the like, the so-called "TDMA (Time Division Multiple Access) system" has been commonly used, in which a carrier wave is switched on and off intermittently to be transmitted in the so-called burst form, and in which a plurality of stations transmit signals in a time division fashion along the time axis. When the synchronized detection system is applied to such communication systems for receiving and demodulating the modulated waves, the necessity for reproducing a reference carrier wave occurs every time the burst signal is received. For that purpose, normally a specific pattern of signal for use in the reproduction of a carrier wave, so-called "preamble work", is added to the beginning of the burst form of input signal. If this additional signal becomes longer, the efficiency of the transmission line is deteriorated, so that the carrier wave reproduction cannot be established within a minimum time. The time needed for the establishment of the carrier wave reproduction is called "acquisition time". It is to be noted that the preamble word generally includes a pattern common to all the stations and a pattern characteristic of the individual station in addition to said specific pattern for the establishment of said synchronizing relationship. If the inverse modulation system is employed as a carrier wave reproducer circuit for the above-described communication system, then in the beginning of the reception of the burst signal, that is, in the acquisition time, the reference carrier wave will not be fully established in this reproducer circuit, so that the N-phase phase-demodulation in the N-phase phase demodulator is incomplete and thus the inverse modulation in the N-phase inverse phase-modulator is also incomplete. Consequently, the inverse modulation system had a disadvantage that the acquisition time was prolonged because of these incompletions. On the other hand, if the frequency multiplier system is employed, though it has the advantage that the incompletion of operation during the acquisition time, as is the case with the inverse modulation system, does not exist, it has the disadvantage that a frequency N times as high as the reference carrier frequency must be dealt with and thus the circuit design is more difficult.